In recent years, along with the wide spread of an optical transmission system for transmitting a data by using an optical signal, a large volume of data has been able to be transmitted. The optical transmission system includes a plurality of nodes connected by optical transmission lines exemplified by optical fibers. Typically, in order to avoid failure such as loss of frame (out of synchronization) of an optical signal or a physical breakdown of the optical fiber, an optical transmission line between the nodes includes duplicated optical transmission lines for a service system and a protection system.
On the other hand, an optical transmission system for an optical communication network includes an optical transmitting & receiving unit mounting station (line concentration station) connected to a host station through an optical transmission line; and a terminal side transmitting & receiving unit provided between a terminal unit and the line concentration station. The terminal side transmitting & receiving unit is connected to the line concentration station through a drop cable (optical fiber). The terminal unit is connected to the optical communication network through the terminal side transmitting & receiving unit mounting station, and communicates with another terminal unit or server in the network.
In such an optical transmission system, a line from a host station to the terminal side transmitting & receiving unit mounting station is preferably made redundant by duplicate or multiple transmission lines. In this case, the optical transmission system is generally provided with a selector for switching the optical transmission line between the host station and the terminal unit. A signal supplied from the host station side through the duplicate optical transmission lines is selected by the selector and then outputted to the terminal unit. Also, a signal supplied from the terminal unit is outputted to the host station side through the optical transmission lines selected by the selector.
The selector typically selects a service system optical transmission line as an optical transmission line for connecting between the host station side and the terminal unit. When any failure has occurred in the service system optical transmission line, the selector switches to a protection system optical transmission line. Also, the selector is required to switch the optical transmission line to be used on the basis of a failure state of the optical transmission line in use as a communication line. For this reason, the above-described optical transmission system needs to be provided with a unit adapted to monitor failure states of the transmission lines.
An example of a technique for monitoring a failure state in an optical transmission system having a redundant optical transmission line, or a technique for switching a transmission line on the basis of a monitoring result is disclosed in the following related arts.
For example, a technique for switching to a protection system optical transmission line in response to a break of an optical fiber is described in Japanese Patent Application Publication (JP-P1992-334135A, related art 1). In the related art 1, each of transmission lines duplicated by a directional coupler is provided with an optical receiver for detecting the break of the transmission line. The optical receiver converts an optical signal into an electrical signal, to detect the break of the transmission line, and outputs an alarm data for controlling a switch for switching the transmission line.
Japanese Patent Application Publication (JP-P2000-269890A, related art 2) describes that if any failure has occurred in a service system transmission line between terminal stations, an optical switch provided in the terminal station having detected the failure switches the optical transmission line between the service and protection systems.
Japanese Patent Application Publication (JP-P2000-358261A, related art 3) describes an optical cross-connector apparatus that monitors a line for an optical signal set in an apparatus without affecting the optical signal (service signal) to be transmitted. In the technique described in the related art 3, connection destinations of a plurality of input side optical transmission lines are selected from a plurality of output side optical transmission lines by an optical switch. At this time, the service signal transmitted in the optical switch is subjected to light intensity modulation by a monitor signal. An optical line monitoring section refers to the monitor signal extracted from a light-intensity-modulated optical signal to identify an optical transmission line selectively connected by the optical switch, and grasps a failure state of each of the optical transmission lines. The optical switch is controlled on the basis of the failure state to determine input and output side transmission lines to be connected.
Also, a technique for switching a transmission line without use of a selector having a switching function is described in Japanese International Patent Application Publication (JP-P2002-523921A, related art 4). In an optical transmission system described in the related art 4, by attenuating a signal in any one of duplicated transmission lines, one transmission line is brought into a standby state, and the other is brought into an operation state. An amplifier on the transmission line in the standby state supplies pumping energy of a low level to an optical fiber doped with fluorescent material. Thus, an incoming optical signal is attenuated by the optical fiber doped with fluorescent material, and amplified natural radiation is generated from the optical fiber. At this time, a pumping laser is current-modulated by a low frequency signal, and the amplified natural radiation is modulated. The modulated natural radiation is detected by a receiver on the transmission line as a control tone.
In an optical transmission system described in the related art 4, the receiver provided on the transmission line monitors a signal level on the transmission line in the operation state to detect a failure having occurred on the transmission line. For example, if the failure is detected by the receiver on the operating transmission line (service system transmission line), an amplifier on the service system transmission line performs the above-described operation, and an amplifier on a transmission line in the standby state (protection system transmission line) is switched to perform a normal operation. Also, when the control tone is not detected on the protection system transmission line, the above switching operation is stopped.
Further, Japanese Patent Application Publication (JP-P2001-44939A, related art 5) describes a technique for grasping an operation state of an optical amplifier on the basis of data from the optical amplifier provided in the middle of an undersea optical fiber cable. An optical amplifier provided undersea described in the related art 5 superposes a state signal or a control signal on an optical signal with low frequency modulation to output the superposed signal to a ground station. At this time, a type of low frequency modulation is set to be different for each optical amplifier and also for each operation state. For this reason, the ground station can grasp a state of each optical amplifier by detecting the type of modulation.
On the other hand, in an optical transmission system in which a terminal unit is connected to a host station through a line concentration station and a terminal side transmitting & receiving unit, an optical transmission line needs to be switched on the basis of a failure state of an optical transmission line between the host station and the line concentration station and a failure state of an optical transmission line between the line concentration station and the terminal side transmitting & receiving unit. That is, in the optical transmission system, the optical transmission line to be used is selected according to the failure states on the upstream and downstream sides of the line concentration station.
However, in any of the above-described related arts 1 to 4, such an optical transmission system is not disclosed that an optical transmission line is selected according to the failure states on the upstream and downstream sides. On the other hand, in the related art 5, a technique for switching an optical transmission line to be used in consideration of failure states of transmission lines on upstream and downstream sides is described. However, it is necessary to provide another transmission line for transmitting the failure states separately from the optical transmission lines. In this case, the transmission line for transmitting the failure state must be provided, and therefore an entire system cost increases. In particular, when a unit for detecting the failure state and a unit for switching the transmission line are far from each other, an increase in cost due to the transmission line for transmitting the failure state becomes significant.